This invention relates to a method for preparing organotin compounds. The invention further relates to a method for preparing diorganotin carboxylates exhibiting reduced odor and fuming, particularly at elevated temperatures.
It is known that the so-called "non-stoichiometric" diorganotin derivatives of carboxylic acids containing more than one mole of tin for each mole of carboxylic acid residues are more stable than the corresponding stoichiometric compounds in that the odor of the carboxylic acid component is considerably less, especially at elevated temperatures. The property is particularly desirable for the diorganotin maleates, since the odor of maleic acid is very disagreeable in addition to being an irritant to the eyes and respiratory tract. U.S. Pat. No. 2,628,211 teaches the preparation of non-stoichiometric diorganotin carboxylates by "polymerization" of the corresponding stoichiometric compound obtained by reacting one mole of a diorganotin oxide for every 2 moles of a monocarboxylic acid or 1 mole of a dicarboxylic acid. The reaction is carried out in a liquid medium at elevated temperatures. "Polymerization" of the resultant stoichiometric compound is achieved by heating the compound in the presence of moisture. The degree of polymerization is dependent upon the specific reaction conditions employed. Some provision must be made for removing the free acid formed as a by-product of the reaction. The examples contained in the aforementioned patent employ a stream of moist air or steam for this purpose. Compounds containing more than three tin atoms per molecule are obtained by passing moist air through the reaction mixture for at least 6 hours, a costly and time-consuming procedure. Since the compound must remain in a reaction vessel throughout this treatment period, the process would not be commercially feasible since it would make only limited use of costly processing equipment, resulting in low volume efficiency in terms of amount of compound produced per unit time. This prior art method requires two steps, namely preparation of the "stoichiometric" diorganotin carboxylate and removal of the water produced as a by-product, followed by oligomerization of the carboxylate with concurrent removal of the by-product acid.
It has now been found that by using a solid acid anhydride instead of the corresponding carboxylic acid the relatively inefficient prior art method for preparing non-stoichiometric diorganotin carboxylates can be replaced by a one step method employing a solid phase reaction that does not generate any by-products. This process can be carried out with a minimum of residence time in the equipment used to blend the reagents.